The pathway from the lateral geniculate nucleus to the striate cortex is reciprocated by a dense descending projection that plays an important role in regulating the activity of geniculate neurons. An important question that remains unresolved about the corticogeniculate pathway is whether it functions as a single system, exerting similar influences over all the neurons that it contacts, or whether it consists of several parallel pathways that can selectively influence different populations of geniculate neurons. The goal of this proposal is to fill this gap in our knowledge by identifying the cell types and patterns of connections that compose the corticogeniculate pathway in the macaque. Four specific aims are proposed: 1) to determine whether neurons projecting to different layers of the lateral geniculate nucleus are segregated within layer VI of striate cortex; 2) to determine whether different types of corticogeniculate fibers can be identified on the basis of their morphology and pattern of termination; 3) to determine whether different classes of corticogeniculate neurons can be defined on the basis of their dendritic morphology and, if so, how these different classes project to the magno- and parvicellular layers of the lateral geniculate nucleus; and 4) to determine whether separate populations of corticogeniculate neurons can be distinguished on the basis of their response properties. The results of the proposed experiments will provide insights into the functional organization of the corticogeniculate pathway by specifying the capacity of the system to regulate selectively the activity of different populations of lateral geniculate neurons. This information is essential for understanding how this pathway contributes to visual perception and may reveal common principles that apply to the corticothalamic pathways in other sensory systems.